1. Field of the Invention
This invention is generally concerned with what are known in the art as high-side driver circuits, in particular for switching power supplies.
2. Description of the Related Art
Broadly speaking, a high-side driver is a circuit which transforms a low-side (low voltage) gate drive signal, generally referenced to a system and/or chip ground, to a high-side (high voltage) gate drive signal which is typically referenced to the mid-point of a half bridge. This mid-point has a potential which can vary between, for example, 0 and 500 volts above the system or chip/PCB (Printed Circuit Board) ground. Often the mid-point potential is rapidly varying over a range of several hundred volts. The high-side driver circuit provides a means of translating or communicating between the low (voltage)-side and the high (voltage)-side circuitry.
FIG. 1 shows an example fluorescent lamp driver circuit 100 incorporating a high-side driver 110.
The lamp driver has a mains power supply input 102 to a rectifier and filter stage 104, for example comprising a bridge rectifier and smoothing capacitors, which provides a low voltage supply to a control circuit 106. The rectifier and filter stage 104 also provides a high voltage supply 109, optionally via a PFC (Power Factor Control) stage 108.
The control circuit provides set and reset outputs to a pair of level shift transistors which provide input signals to high-side driver 110. The control circuit also provides a low-side drive signal to a low-side gate drive circuit 114. The high-side drive 110 drives a high-side switching transistor 116a and the low-side gate drive drives a low-side switching transistor 116b. Transistors 116a and 116b form a half-bridge coupled between the high voltage supply and a system ground rail 118. The high-side driver 110 is connected to a mid-point 120 of the half-bridge. This mid-point also provides a high (alternating current) voltage output to lamp and resonant capacitor 122 via a de blocking capacitor and resonant inductor 124.
In operation the low-side drive signal typically comprises a waveform with a duty cycle of around 40 percent. Similarly the set and reset signals are controlled so that the drive to the high-side transistor also has a duty cycle of around 40 percent. The low-side and high-side transistor drive waveforms are aligned so that the low-side transistor and high-side transistor are not on simultaneously, the less than 50 percent duty cycle waveforms providing a degree of tolerance in achieving this.
A difficulty with design of the high-side driver is that the signal (usually in the form of a current) which is used to communicate between the low-side and high-side circuitry is easily confused by system noise, especially when the half-bridge is switching.
Conventional solutions to this problem rely on either making the communicating signal very large relative to the interfering signals, with a consequent waste of power, or on using blanking signals which blank out the (regular) interfering signal. As the interfering signals happen at known times, in particular when the mid-point of the bridge is flying (transitioning) high or low, blanking pulses can be used to blank out these signals, but this approach also wastes power, is tricky to implement, and is prone to error. Background prior art can be found in: U.S. Pat. Nos. 4,994,955; 6,897,492; 6,781,422; 6,781,423; 5,274,274; 5,068,571; and WO2003/055072.